High-speed, directly modulated semiconductor ridge waveguide lasers are well known in the art. “Directly modulated”, as used in this specification, relates to lasers where the source current is modulated in time. “High-speed”, as used in this specification, relates to lasers functioning at a frequency of 1 GHz or higher. FIGS. 1A and 1B illustrate a top view and a cross-sectional view, respectively, of a conventional high-speed, directly modulated ridge waveguide laser chip. The laser chip 100 comprises a substrate 102 with a mounting surface 110 and a junction surface 112. At the mounting surface 110, the laser chip 100 is eventually coupled to a submount, a printed circuit board, or some other surface. The laser chip 100 is thus mounted “junction side up”. The junction surface 112 comprises a contact pad 108 and a ridge structure 104. Typically, the ridge structure 104 protrudes beyond the edge of the junction surface 112.
In large scale manufacturing of laser chips, a multitude of laser chips are formed on a semiconductor wafer. Each of these chips are then handled mostly with pick-and-place tools 106. Most pick-and-place tools possess vacuum collets for holding the laser chip. FIG. 2 illustrates a conventional pick-and-place collet. The collet typically comprises a shank 202 and a tip 204. The inner diameter of the tip 204 can be approximately 152 microns. To pick up a laser chip, a vacuum is created within the collet. The vacuum is released to place the laser chip 100.
However, because the ridge structure 104 protrudes beyond the edge of the junction surface 112, the ridge structure 104 suffers from mechanical damage due to the impact from the tool 106. This can render many laser chips on a wafer non-functional, adversely affecting the device yield of the wafer.
One approach for lower-speed lasers is described in U.S. Pat. No. 5,305,340. FIG. 3 illustrates this conventional approach for a “junction side down” mounted laser. The laser includes a substrate 30, cladding layers 32 and 36, an active region layer 34, and a ridge part 38. A protective material 40, specifically gold, are disposed on the surface of the active region layer 36. Protective walls 48, comprising gold as well, are formed on either side of the ridge element. These walls 48 prevent damage to the ridge element, whether the laser is mounted junction side down or up.
However, this conventional approach is problematic for high-speed, directly modulated ridge waveguide lasers. Due to the small size of such lasers, the connection of the walls 48 to the material 40 on the active region layer 36 would cause problems with parasitic capacitance at the contacts (layer 40). To properly control parasitic capacitance, contacts to the active region must be sufficiently small. For high-speed, directly modulated ridge waveguide lasers, this cannot be accomplished when the conductive material on the active region layer is connected to the conductive materials of the walls 48.
Accordingly, there exists a need for a method and apparatus for preventing damage to a ridge of a high-speed, directly modulated ridge waveguide laser during manufacturing. The method and apparatus should be easy and cost effective to implement, and be easily practiced with standard pick-and-place tools. The method and apparatus should also allow for the proper control of parasitic capacitance. The present invention addresses such a need.